1. Field of the Invention
The present invention relates in general to a rotary cutting tool, and is more particularly concerned with a formed rotary cutting tool having peripheral cutting edges whose diameter varies in the axial direction.
2. Discussion of the Related Art
In a highly-efficient multi-utility plant, an exhaust heat recovery plant for energy conservation or saving, or other kinds of plants, there is employed a large-scale high-power gas turbine. FIG. 6 is a view showing a part of one example of the gas turbine having a rotary shaft 10 and a multiplicity of blades 14 which cooperate with each other to form a wheel of the turbine. In the outer circumferential surface of the rotary shaft 10, there are formed the same number of grooves 12 as that of the blades 14. Each blade 14 is fitted at one of its opposite longitudinal ends in the corresponding one of the grooves 12. FIG. 7 is an enlarged view in cross section showing each groove 12, which is taken in a plane perpendicular to the axial direction of the rotary shaft 10. The groove 12 has in the cross section a Christmas tree-like shape which is symmetrical with respect to its laterally or widthwise central line S. The width of the groove 12 generally decreases in a radially inward direction from the outer circumferential surface of the rotary shaft 10 towards the axis of the rotary shaft 10, i.e., in the downward direction as seen in FIG. 7. More specifically described, the groove 12 has three laterally enlarged portions 16, 18, 20 which are spaced apart from each other in its depth direction, and which have larger width values than the adjacent portions. Of the three laterally enlarged portions 16, 18, 20, the portion 16 is located nearest to the circumferential surface of the rotary shaft 10, while the portion 20 is located nearest to the axis of the rotary shaft 10, with the portion 18 being located therebetween. The groove 12 has the largest width at the portion 16. Namely, the width of the groove 12 at the portion 16 is enlarged more than at the other two portions 18, 20. The width of the groove 12 at the portion 20 is enlarged less than at the other two portions 16, 18.
The above-described groove 12 having the Christmas tree-like configuration or profile is, in general, formed by a plurality of ordinary end mills or other rotary cutting tools each having a cutting edge or edges on its outer circumferential surface. These rotary cutting tools rotated about the axis are fed in a direction or directions perpendicular to the axis of the cutting tool for thereby forming the groove 12. For example, those cutting tools are used sequentially in a plurality of rough cutting steps (a)-(d) and a finish cutting step (e). The rough cutting steps (a)-(d) are implemented on the workpiece in this alphabetical order, so that a part or parts shaded in FIGS. 8 is cut off or removed in each rough cutting step. The rough cutting steps (a)-(d) are followed by the finish cutting step (e) in which an inner surface of the rough-cut groove is finished, to form the groove 12.
However, this conventional method of cutting or forming the groove 12 requires, as described above, the plurality of cutting tools having different dimensions and configurations and also the plurality of cutting steps, resulting in a prolonged machining time and an accordingly increased machining cost.